LNG (liquefied natural gas) generally refers to colorless, transparent cryogenic liquid converted from natural gas (predominantly methane) that is cooled to approximately −162□ and condensed to 1/600th the volume.
As LNG emerges as an energy source, efficient transportation means have been sought in order to transport LNG from a supply site to a demand site in a large scale so as to utilize LNG as energy. Resulted in a part of this effort is LNG carriers, which can transport a large quantity of LNG by sea.
LNG carriers need to be furnished with a cargo that can keep and store cryogenically liquefied LNG, but such carriers require intricate and difficult conditions.
That is, since LNG has vapor pressure that is higher than atmospheric pressure and boiling point of approximately −162□, the cargo that stores LNG needs to be constructed with materials that can withstand very low temperature, for example, aluminum steel, stainless steel and 33% nickel steel, and designed in a unique insulation structure that can withstand thermal stress and thermal contraction and can be protected from heat leakage, in order to keep and store LNG safely.
Described below with reference to the accompanying drawings is the insulation structure of a conventional LNG carrier cargo.
FIG. 1 is a sectional view illustrating a conventional insulation structure of an LNG carrier cargo. As illustrated, a bottom insulation panel 10 is adhered and fixed by way of a fixing plate 10a to an internal face of a hull 1 of an LNG carrier by epoxy mastic 13 and a stud bolt 14.
Here, interposed and adhered in between the bottom insulation panel 10 and a top insulation panel 20 is a rigid triplex 22. When the bottom insulation panel 10 is adhered to a cargo wall, the bottom insulation panel 10 is formed with a gap 40 so that a flat joint 18 made of a glass wool material can be inserted in the gap 40 formed between bottom insulation panels 10.
Then, a top bridge panel 28 is attached in between the top insulation panels 20 by adhering a supple triplex 26 over the rigid triplex 22, which is already attached, with epoxy glue 24 and then adhering the top bridge panel 28 over the supple triplex 26 with epoxy glue 24.
The top insulation panel 20 and an upper part of the top bridge panel 28 have a same planar surface, on which a corrugated membrane 30 is attached by way of an anchor strip 32 to complete the cargo wall.
Looking at how the internal face of the hull 1 and the bottom insulation panel 10 of an LNG carrier are assembled in further detail, the stud bolt 14 is adhered to an inner wall of the hull 1 by resistance welding, and a hole, through which the stud bolt 14 can be inserted, is pre-formed in the bottom insulation panel 10. Accordingly, assembly is completed by engaging a nut 14a with the stud bolt 14 and inserting a cylinder-shaped foam plug 15 in the hole formed in the bottom insulation panel 10.
As corner areas of the cargo of the conventional LNG carrier need to be made more rigid than other flat areas, the structure of a corner of the cargo of the LNG carrier will be described below with reference to the accompanying drawings.
FIG. 2 is a sectional view illustrating a structure of a cargo insulation corner of an LNG carrier in accordance with a conventional embodiment of U.S. Pat. No. 6,035,795.
As illustrated, two sheets 51 of insulating material intersect each other to form the corner of the cargo, and installed on an internal side toward the inside of the cargo at a region where these sheets 51 intersect is an insulating sheet 52, which is attached in between two wooden boards 53. In order to prevent a secondary barrier from cracking due to deformation of the hull and thermal deformation caused by the cryogenic LNG, the wooden boards 53 are used for the corner area, unlike the flat areas.
FIG. 3 is a sectional view illustrating a structure of a cargo insulation corner of an LNG carrier in accordance with another conventional embodiment of U.S. Pat. No. 6,378,722.
As illustrated, a flexible gasket 62 is installed at an intersecting region of insulation layers 61 that corresponds to a corner area of the cargo, and corrugations (not shown) are formed in a primary barrier (not shown) in order to prevent stress caused by thermal contraction from converging at the corner area, thereby reducing the stress applied to the corner area.
Referring back to FIG. 1, the corrugated membrane 30, which is the primary barrier, is directly contacted with LNG. In a large capacity cargo, the LNG inside the cargo may slosh, thereby applying pressure to the cargo, if the LNG carrier is rolled or pitched due to the waves or winds.
The pressure caused by sloshing affects the corrugated membrane 30, which is in direct contact with LNG, and the top insulation panel 20, which is in contact with the corrugated membrane 30. Here, if the impact load and stress caused by the pressure exceed the rigidity of the corrugated membrane 30 and the top insulation panel 20, plastic deformation and crack may occur, lowering the safety of the LNG cargo.
Particularly, a joint area of the corrugated membrane 30, which is the primary barrier, and the top insulation panel 20, which is the insulator, is more vulnerable to the impact load and stress caused by the deformation and sloshing of the hull.
As described above, the structure of the corner area of the cargo of the LNG carrier in accordance with the conventional art has been constructed rigidly by use of thick plywood, called hard-wood key, or has been corrugated to reduce the stress. However, as the structure is non-continuous, the stress generated due to the sloshing, the deformation of the hull and the change in temperature converges at the corner area. Moreover, it is difficult to undertake the construction of the secondary barrier since the corner area forms an acute angle, and the weight is greatly increased since a material such as plywood is used.